1) Field of the Invention
The present invention relates to a stabilizing method of isocyanate compounds to suppress both yellowing and the occurrence of turbidity, which would otherwise take place due to changes along the passage of time and also to isocyanate compositions stabilized by the above method. The isocyanate compositions according to the present invention are particularly useful as raw materials for optical urethane resins for plastic lenses and the like, which resins are required to have colorless transparency.
2) Description of the Related Art
Isocyanate compounds are accompanied by the drawback that, because of the high reactivity exhibited by each isocyanato group thereof, they lack stability and tend to undergo coloring and/or self-polymerization during storage. Addition of a stabilizer is indispensable to suppress coloring and self polymerization.
Various compounds have heretofore been known as stabilizers for isocyanate compounds. Representative ones among them are phenols such as 2,6-di-tert. butyl-p-cresol (U.S. Pat. No. 3,715,381) and phosphite esters such as triphenyl phosphite (Japanese Patent Publication No. 33438/1970). In addition, many other stabilizers have also been known, including ureas, carbamates, acid amides (Japanese Patent Publication No. 7044/1970, Japanese Patent Laid-Open No. 36546/1975); acid substances such as perchloric acid and trifluoromethanesulfonic acid (German Patent Publication No. 2,837,770); carbon dioxide and sulfur dioxide (U.S. Pat. No. 3,247,236); organic amines (Japanese Patent Laid-Open No. 101344/1975); acid chlorides (Japanese Patent Laid-Open No. 179917/1988); siloxanes (Belgian Patent Publication No. 858,921); organotin compounds (European Patent Publication No. 203,874); and acyl isocyanate compounds (Japanese Patent Publication No. 3825/1973).
Known examples of phenolic stabilizers useful in the practice of the present invention include pyrogallol, catechol, guaiacol, eugenol, pentachlorophenol (Japanese Patent Publication No. 3825/1973, Japanese Patent Publication No. 7044/1970), 2,6-di-tert. butyl-p-cresol and methacresol (Japanese Patent Laid-Open No. 48619/1976), as well as alkyl-substituted phenols and bisphenols (Japanese Patent Publication No. 27,594/1971, Polish Patent Publication No. 100,154, British Patent Publication No. 1,315,520, U.S. Pat. No. 3,682,902, Japanese Patent Laid-Open No. 98050/1984).
It is, however, still difficult to suppress both the coloring and self-polymerization-induced turbiding of isocyanate compounds even by these stabilizers. Satisfactory results have not yet been obtained.
It is known that aliphatic polyisocyanates, particularly xylylene diisocyanate tend to become turbid due to the formation of a 1-nylon-type polymer through self-polymerization [Journal of the Chemical Society, 82, 866 (1960)]. It has therefore been found that the stabilizers disclosed in such patent publications do not permit stable storage over 3 months or longer at room temperature.
There is hence an outstanding demand for the development of a stabilizer which can suppress both yellowing and turbiding of isocyanate compounds over a long period of time.